1. Field of the Invention
This invention relates generally to the reduction of total airplane drag. In particular, it relates to reducing total airplane drag by changing high drag turbulent wing leading edge flow to low drag laminar wing leading edge flow in supersonic flight regimes.
2. Description of Related Art
The destabilizing effects of leading-edge sweep on the laminar wing boundary layer at subsonic speeds have been known since the early 1950s. The dominant transition mechanisms are "attachment-line contaminations" and "crossflow instability." The problem of turbulent contamination along the attachment line of swept wings had to be addressed in the 1960s before laminar flow could be achieved at high Reynolds numbers on swept wings. (See, Pfenninger, W.: Flow Phenomena at the Leading Edge of Swept Wings. Recent Developments in Boundary Layer Research--Part IV, AGARDograph 97, May 1965; Pfenninger, W.: Laminar Flow Control-Laminarization, AGARD-R-654, March 1977, pp. 3-1-3-75; and Gaster, M.: On the Flow Along Swept Leading Edges. The Aeronautical Quarterly, Vol. XVIII, Part 2, May 1967, pp. 165-184.) In some of these studies, leading edge contamination was minimized by strong local suction and leading-edge fences with suction. In others, a faired bump protruding from the leading edge of a wind-tunnel model wing was used to prevent the spanwise propagation of turbulence along the attachment line. Wind tunnel tests of this bump attached to a cylindrical model indicated that for subsonic flight, laminar flow could be maintained at sweep angles of 60.degree..
Evidence of crossflow instability was first observed in flight experiments on the leading-edge portions of swept wings as regularly spaced streaks caused by variations in mass transfer from surface coatings. Gray, W. E.: The Nature of the Boundary-Layer Flow at the Nose of a Swept Wing. RAE TM Aero 256, 1952. The streaks were aligned in the local streamwise direction and were present over the upstream region preceding transition on swept-back wings. These streaks were caused by co-rotating vortices arising from the inflectional instability of the crossflow boundary-layer profiles in the upstream region of the swept wings. Gregory, N.; Stuart, J. T.; and Walker, W. S.: On the Stability of Three-Dimensional Boundary Layers with Application to the Flow Due to a Rotating Disk. Philos. Trans. Roy. Soc., London, Ser. A, Vol. 248, No. 943, 1955, pp. 155-199.
In flight, spanwise contamination may be minimized by either active or passive means. The cheapest method for avoiding turbulent attachment-line flow would be a passive device to either prevent turbulence from spreading down the leading edge, or to relaminarize an already turbulent boundary layer. Recent subsonic experimental evidence suggests several passive devices to relaminarize leading edge-flow in the subsonic region. Seyfang, G. R.: Turbulent Drag Reduction by Passive Means. Proceedings of the International Conference, London, England, Sep. 15-17, 1987, Vol. 2, London, Roy. Aero. Soc., 1987, pp. 568-601. Seyfang's wind tunnel results for a swept-wing model covered sweep angles of 25.degree. to 75.degree. at M.sub..infin. =0.25.
That such devices should have utility in the supersonic region is not to be expected by those of skill in the art, as it is well known that a device which is useful in the subsonic region is often not useful in the supersonic region. Moreover, it would appear that when operating in the supersonic region, a boundary layer should become more turbulent as a result of employing such devices.